The signal is fine through all the filtering and amplifying... but where it goes into the U102 hybrid resistor network... I see the signal going into pin 15, but absolutely nothing coming out pin 21. I was bummed - another user mentioned in another post that 606 errors are usually the hybrid resistor network, and that part is $191 from Keysight
There was supposed to be 100k ohms between those two pins, and I was getting 58k - but didn't quite make sense why I was seeing nothing on the output. I guessed the hybrid resistor chip had failed internally... so I desoldered it. And when I tested it, everything was fine - all resistances were dead on accurate.
So... it pretty much has to either be the U411 mux/demux chip or either the U402 or U420 op-amps. I ordered all three parts and will just replace them all.
The parts I am working on/replacing are all located under the metal shield of the meter. I've been touching various parts and now that I desoldered U102, there's flux residue and will be more once I swap out the other 3 parts. Are there any special steps required to maintain accuracy after I button it all back up? Of course, I will clean of the flux residue but is there anything else that needs done? Or is it just a simple matter of calibration?
No wonder, that you measured 58k instead of 100k.. I assume in unpowered state..
There are several resistors in parallel to the 100k+R440, namely the network R406, R403, R405, R407 and further, plus the chain R197, R196, and the 200k of U102-B, and several OpAmp parasitic outputs..
So it is no wonder, that the network miraculously was fine...
Who the heck claimed that the resistor network would fail (often)??
Please show the reference, or who this user is..
And I really doubt that, as it is not evident, how this network should be damaged, or by which mechanism it should fail..
And it's no wonder at all, that you don't see any signal on pin 21.. that is a virtual ground, i.e. always having precisely zero volt!!
Therefore, your assumption, that U411, U420 and U402 failed, is also erroneous..
Why do you think so??
You should see a triangular integration signal on pin 6 of U402-A.. then it's fine..
Only if you don't see that, that may be defect, but only if the other OpAmps, I've mentioned are ok.
I propose you better measure again the A/D and reference OpAmps, before you wildly replace components!
That also could greatly deteriorate the A/D performance, i.e. linearity and precision, as these components may have been selected.. and you are fiddling/soldering at a very delicate circuit part!
Frank